Electrical heating method



March 6, 1945. I c 5 S 2,370,624

ELECTR ICAL HEAT ING METHOD Filed Oct 1, 1942 Summer 21 Henderson, (ZGLLZespie Gttorncu Patented Mar. 6, 1945 UNITED STATES PATENT OFFICE 2,370,824 ELECTRICAL HEATING METHOD Henderson 0. Gillespie, Moorcstown, N. J assignor to Radio Corporation of America, a corpontion of Delaware Application October 1, 1942, Serial No. 460,338

up between the electrodes causes the work to heat up. In many cases, the work is not of uniform cross section throughout its length, and if the electrodes are in engagement with the work at all positions thereof,' non-uniform heating results, the thinner portions of the work becoming hotter than the thicker portions. This condition is highly detrimental in many applications and must be avoided if satisfactory results are to be obtained.

The primary object of my present invention is to provide an improved method of heating dielectric materials with theaid of a high frequency electric field, which method will not be subject to the aforementioned disadvantage.

More particularly, it is an object of my present invention to provide an improved heating method as aforesaid by means of which uniform heating tions thereof and thus heat the work-piece uniformly throughout.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claim. The invention itself, however, both as to its organization and -method of operation, as well as additional ob- .iects and advantages thereof, will best be understood from the following description, when read in connection with the accompanying drawing wherein Figure 1 is a side elevation of a wedge-like Figure 3 is a view similar to Figure l but with the spacing of the electrodes relative to the work indicated thereonas derived from Figure 2-,

Figure 4 is a view similar to Figure 3 but showing the electrodes in proper relation tov the work,

95 as derived from Figure 3, to insure uniform heating thereof, v

Figure 5 is a view representing an elemental volume of work-piece and surrounding dielectric between the two electrodes at any random point of a work-piece of non-uniform cross section will through the cross section of the work, this view be obtained.

Otherwise stated, it is an object of my present invention to provide an improved heating method which will produce uniform generation of heat in a dielectric material of non-uniform cross section.

It is also an object of this invention to provide an improved method as aforesaid which is highly efficient in practice.

In practicing this invention, the electrodes are shaped and so disposed relative to the work as to produce uniform current flow in all parts thereof. To determine the proper shape and position of the electrodes relative to the work, the dimensions of the latter are redrawn t a new thickness scale determined by dividing all thickness dimensions thereof by the relative dielectric constants of the material of the work-piece and the material of the surrounding dielectric (usually air). Electrodes then spaced from the original workpiece according to the difference in distance be- -tween two parallel lines drawn through the thickbeing referred to hereinafter in explaining the theory involved in my invention,

Figure 6 is a view similar to Figure 4 but showing the electrodes applied to a non-uniform dielectric work-piece of other than wedge shape,

and

Figure 7 is a central sectional view of a modification of the invention.

Referring more particularly to the drawing,

40 wherein similar reference characters designate j corresponding parts throughout, there is shown a wedge-shaped work-piece l of dielectric material, the piece I having a thickness ii at oneend, where its cross sectional dimension is greatest,

5 and tapering down gradually to a thickness ta at I would become hotter in the region of its thin- Y nest end than in the region of its thickest end. It is this condition which the present invention obviates in the following manner:

As the first step in practicing this invention,

the work-piece I is redrawn or otherwise suitably reproduced in the manner shown in Figure 2 with its length the sam as the original and all its cross-sectional or thickness dimensions that fraction of the original thickness as is the ratio of the dielectric constant of the medium which surrounds the work-piece I to the dielectric constant of the material of the work-piece. Since the surrounding medium is usually air, the dielectric constant of which is unity, if it is assumed that the dielectric constant of the workpiece material is 2, then the aforementioned fraction becomes In other words, for the assumed case, the work-piece is redrawn to a thickness of one-half the original thickness, so that the thickest end of the redrawn piece is and the thinnest end thereof is by way of illustrationv only, but this is not to be' construed as being a necessary or required limitation. In any case, the lines X and Y are arranged. parallel to each other in Order to provide therebetween an infinite number of ele mental volumes of work-piece and adjacent air portions any one of which volumes may be chosen at random and will be equal to any other such volume of work-piece and adjacent air portions. These elemental volumes are referred to in greater detail hereinafter in explaining the principle involved in the present invention.

After the lines and Y have been drawn as above described, the distances DI, D2, D3 D6 are read off between each of the lines X and Y and the adjacent surfaces In and lb, respectively, at a plurality of spaced points along the length of the redrawn piece. are then laid off from the surfaces I a and lb at corresponding points along the length of the piece I drawn to original dimensions, as shown in Figure 3. The points so laid oil define the paths along which the electrodes should lie, and a pair of electrodes I and 9 so disposed and adapted to be connected to a source of high frequency power, such as a radio frequency uscilla tion generator I not shown), by a concentric line II is shown in Figure 4. This arrangement of electrodes relative to the work will insure uniform heating of the work at all points thereof.

For an understanding of the principle involved in the abov arrangement, reference will now be made to an elemental volume V (Figs. 4

:and 5) of the work-piece I and adjacent air elements I3 taken at random through the workpiece between the electrodes and 9. Let it be assumed that the cross sectional area of the elemental volume V is dA, that the net capacity of the two air elements or particles I3 is (A, and that the capacity of the elemental work-piece I in the volume V is d0. The resultant series These distances capacity d6 of the elemental volume V is given by the equation HGHEZWEZ If the capacity dC is everywhere the same for each elemental volume V, it is obvious that the same current will flow through each elemental volume, and therefor uniform heating of the work-piece will result. -Now if -=the dielectric constant of the, work-piece I, Kl=the dielectric constant of air (unity) t..=the thickness of the work-piece, ts=the thickness of the air, and

a=a proportionality factor,

then

K-dA (Ice- (2) and K.dA dA n. "at: 3)

Substituting Equation Z'and 3 in Equation 1,

1 a t, i review?) 5) Now, if, at every point in the work, we make Tt,. T t, I T. K where T is the maximum thickness of the work, then, substituting Equation 6 in Equation 5,

In other words, the capacity of each elemental volume V is the same as at the thickest part of the work where the electrodes are in contact with it, which means that each elemental volume oifers the same impedance to the flow of current therethrough as does the thickest part of the as it approaches its tip l9. Were the e'lectrodesf, I and 9 "form fitted to the blade I5, so ltospeak',

it is obvious that the portion thereof in ,.the region of its tip I9 would heat much more rapidly than the base portion and might even char before the base portion II would be heated sufficiently. However, by placing the electrodes 1 and 9 in engagement with the thick, base portion I! only and from there on spacing the elec" trodes from the blade in the manner heretofore acme described, uniform heating at all parts of the blade will be insured.

In the modifications thus far described, the work-piece l is assumed to be immersed in air, and, with the electrodes spaced therefrom as above described, no pressure can be applied to the work, as is sometimes necessary during the heating operation. To meet this situation, the

, work-piece I may be placed between or surrounded by a second, low-loss, dielectric material 2! and the electrodes 1 and 9 placed against the material 2|, as shown in Figure 7. The material 2| may be porcelain, for example, or any one or more of a number of other suitable materials.

which can withstand the required pressure, such as those known commercially as Isolantite and Mycalexf In any event, the thickness dimensions of the material 2| should be determined similarly to the air thickness represented by the distances DI, D2 D6 in Figures 2 and 3. If

found desirable, the material 2| may space the electrodes 1 and 9 from all points of the workpiece I along its entire length, as shown in Figure 7, but if it is suitable to place the electrodes in direct engagement with the work, then this should be done only at the thickest portions thereof and the material 2| used to fill in the spaces left between the electrodes and the work at all other Points.

Although I have shown and described several forms of my invention and several ways in which it can be carried into practice, it will be obvious,

tothoseskilledin'theartthatmanyothersare also possible. I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claim.

I claim as my inventionz.

In electrical heating apparatus for heating a dielectric work-piece of non-uniform cross section throughout its length and embraced by a second dielectric material having a different dielectric V constant than the material of said work-piece by subjecting said work-piece to a high frequency electric field between a pair of cooperating electrodes, the method of positioning said electrodes relative to said work-piece which comprises first reproducing said work-piece to the same length dimension as said original work-piece but to that fractional cross-sectional dimension thereof which, is the ratio of the dielectric constant of said'second dielectric material to the dielectric constant of said work-piece material, then dis- "posing two parallel lines which pass through only the thickest portions 'of said reproduced piece, then measuring 011 ata plurality of spaced 'points longitudinally of said reproduced piece 

